Vaisala DMT143 - Manual (Page 17)

Chapter 3 _______________________________________________________ Functional Description
VAISALA ________________________________________________________________________ 15
CHAPTER 3
FUNCTIONAL DESCRIPTION
This chapter describes the advanced functionality of DMT143, including
the auto-calibration, sensor purge, and sensor warming functions.
DRYCAP
®
Technology
Vaisala DRYCAP
®
dewpoint measurement technology ensures accurate
measurement with excellent long term stability. This results in very low
maintenance requirements for the transmitter. The lasting performance is
achieved with microprocessor technology and software that automatically
performs self-diagnostic functions in addition to the normal dewpoint
measurement. The self-diagnostic procedures that are conducted are
called auto-calibration, sensor purge, and sensor warming.
Auto-Calibration
The auto-calibration feature of the DMT143 transmitter is an automatic
procedure which greatly reduces the possible drift in the dry end of the
dewpoint measurement. It is performed at one hour intervals, and when
the power is switched on. During auto-calibration the sensor is warmed
for a short period (< 1 min) and the sensor capacitance values are
evaluated at the elevated temperature. The possible dry end drift is then
corrected to correspond to the calibrated values. During the auto-
calibration the transmitter outputs the T
d
value prior to the procedure.
Auto-calibration is carried out only if several criteria for the
measurement environment are fulfilled. This ensures the reliability of the
adjustments, and maintains the excellent long term stability that the
patented technology offers. These criteria include for example a stable
enough moisture level in the measured atmosphere. If the conditions are
not fulfilled, the auto-calibration function is postponed until satisfactory
conditions are reached.